1. Field of the Invention
The present invention pertains to wide-azimuth, towed array, marine seismic surveys.
2. Description of the Related Art
This section of this document introduces various aspects of the art that may be related to various aspects of the present invention described and/or claimed below. It provides background information to facilitate a better understanding of the various aspects of the present invention. As the section's title implies, this is a discussion of “related” art. That such art is related in no way implies that it is also “prior” art. The related art may or may not be prior art. The discussion in this section of this document is to be read in this light, and not as an admission of prior art.
Illuminating reservoirs with traditional marine seismic acquisition has always been challenging. Much of the challenge has to do with the variation, or lack of it, in the azimuths of the recorded seismic ray-paths. One relatively new acquisition technique to overcome this difficulty is wide-azimuth (“WAZ”) surveying. WAZ surveying is aimed at addressing the illumination problems inherent in traditional narrow-azimuth marine seismic. WAZ surveys provide a step-change improvement in imaging. These types of survey improve signal-to-noise ratio and illumination in complex geology and provide natural attenuation of some multiples.
Wide-azimuth towed-streamer surveys are acquired with multiple vessels. A typical configuration consists in two streamer vessels and two source vessels. On each vessel there is a source array and the sources fire sequentially. One implementation of wide-azimuth acquisition using this vessel configuration is to shoot in one direction with 1200 m sail line interval and then shoot in reverse direction, interleaved lines, at 1200 m sail line interval. N. Moldovenau & M. Egan, 2007, “Some Aspects of Survey Design for Wide-Azimuth Towed-Streamer Acquisition”, SEG, 2007, Extended Abstracts, 56-60. This method of shooting produces shot lines at 600 m, a wide-azimuth distribution and a maximum crossline offset of 4200 m.
More particularly, the basic configuration that is used today by WesternGeco, the assignee hereof, and other companies for wide-azimuth (“WAZ”) acquisition consists of two streamer vessels and two source vessels. The vessels are all aligned at the leading edge of the spread and each of the vessels tows a seismic source. The vessels' separation is 1200 m and the spread width—the crossline distance from the portside streamer to the starboard streamer in a towed streamer array—is 1200 m.
All 4 vessels sail in one direction in a first pass with an interval of 1200 m between them—that is, with a 1200 m sail line interval. The vessels sail also in the opposite direction in a second pass over the survey area. The second pass is also-made using a 1200 m sail line interval. However, the sail lines of the second pass are interleaved in the sense that the vessels in the second pass traverse sail lines that are between the sail lines of the first pass. The number of sail lines that will be executed depends on the size of the survey, for example, twelve sail lines in two passes, or six sail lines in each direction.
As noted above, each of the vessels tows a seismic source. The “shot line interval” is the interval between the seismic sources. The sources are “shot” during each pass. Considering collectively the shooting in the two interleaved directions, the shot line interval is 600 m. This means that the crossline sampling is 600 m. If each seismic source fires sequentially at 37.5 m in conventional fashion, the shot interval along each source line is 150 m (37.5 m*4). This means that the inline shot sampling is 150 m. Note: if two sources are fired simultaneously, the shot interval (the inline sampling) can be reduced by 75 m.